In the manufacture of printed circuit boards or integrated circuits, it is often required to apply a liquid coating to the surface of substrates of various forms including circuit boards, or IC wafers (IC) and flat panel displays (FPDs) In manufacturing printed circuit boards, a photoresist is used to outline the circuit structure onto the copper surface of the board. This is traditionally done by exposing certain segments of the photoresist to ultraviolet radiation using a mask which outlines the circuit structure. The ultraviolet radiation hardens the exposed photoresist. The remaining unexposed photoresist is then washed off. An etchant is then applied to the circuit board which etches off all the copper surfaces that are not protected by the hardened photoresist. The exposed photoresist is then stripped away exposing the copper circuit lines underneath. FPDs use a similar process, however the copper layer is not present and the FPD features have a smaller geometry than circuit boards.
Typically, a number of different layers are applied to a substrate to properly develop the surface of the substrate. Photosensitive chemicals are applied by a variety of methods and then typically baked off to dry the solvents creating a photosensitive film. This film is then exposed to UV (ultraviolet) having a wavelength which can vary depending upon the sensitivity of the material. The UV light is usually transmitted to the film employing a patterning device such as a stepper, scanner, or mask aligner to expose selected portions of the photosensitive coating. The next step is to develop the exposed portion of the film. In this case the "film" is a variety of photoresist.
Developing involves contacting the exposed film or surface with one or more of a variety of different chemicals depending on the type of photoresist used. Use of positive photoresist yields a positive image of the stencil employed in the mask or patterning device, and use of negative photoresist yields a negative of this image. Although different chemicals are employed for developing the different types of photoresist the mechanism for developing is similar.
Prior art methods for accomplishing the developing process include spin developing and use of a dip tank or batch developing. Both methods experience problems which are discussed below.
Spin developing involves placing a substrate on a turntable or other rotating mechanism and applying developer either by dispensing developer onto the center of the substrate thereby causing the fluid to spread out across the substrate through the spinning action, or by spraying developer directly onto the surface of the substrate. The velocity of rotation can be set such as to spread out the developer as evenly as possible and ultimately increased to the point where some loose unreacted developer is forced off the substrate by centrifugal force.
A rinsing process follows the application of developer fluid and is pursued in much the same manner. Rinsing fluid is applied either to the center of a rotating substrate or sprayed on the substrate as with the developer. Rotation velocity of the substrate can be adjusted initially to spread out the rinsing fluid and later increased to the point where the rinsing fluid is forced off the substrate. The rinsing fluid performs three main functions upon being spread around the substrate: it stops the chemical reaction induced by the developer fluid, and acts to wash away unused or unreacted developer. The rinse fluid also helps to wash away "developed" or removed material. The quality and uniformity of the developing process depends, among other things, upon the reaction time between the developer and the photoresist, or other film, and the consistency in this reaction time across the substrate being developed.
A drawback to spin developing is non-uniformity of the development process. It is difficult to ensure even distribution of the developer and rinse fluids and exposure time using spin developing and therefore difficult to achieve uniform develop results on the surface of the substrate. In practice, it has been observed that substrates are usually over-developed toward the outside or outermost perimeter of substrates. Therefore it is a problem in the art that spin coating does not provide for uniform developing of substrates. Spin developing also requires dispensing a substantial quantity of fluid which may never contribute to the developing process, such fluid having been either spun off the substrate before having had a chance to react.
Another drawback to spin developing, particularly for very large substrates, is the significant centripetal forces that are encountered, which can damage certain microscopic structures on the substrate (particularly high aspect ratio features (tall and skinny)).
Another prior art method for developing substrates is batch developing, wherein an entire substrate is dipped into a bath of developer fluid for a prescribed period of time and subsequently into a rinse bath or series of rinse baths. For any given substrate, there may be some improvement in developing consistency across that substrate as compared with spin developing However, the process as a whole suffers from the fact that the submersion of a substrate into the developer bath leaves material in the bath and partially contaminates that bath. This contamination effect progresses with each successive substrate thus making the developer less and less effective with succeeding substrate. Consistency from one substrate to another will therefore not be consistent. It is therefore a problem in the art that batch developing causes inconsistencies between successive substrates developed using the same developer bath.
Therefore it is a problem in the art that the distribution of developer across the surface of a substrate is not even when spin developing.
It is a further problem that chemical reaction time between developer fluid and the substrate is not even across the surface of a substrate when spin developing.
It is a still further problem in the art that the extent of chemical reaction between the developer and film on the substrate is not uniform when spin coating.
It is a still further problem in the art that there is inconsistency in developing between successive substrates developed using the same baths of developer and rinse fluids.
It is a still further problem in the art that centripetal forces during spin developing can damage some of the microscopic features present on the substrate.